In the field of medicine, there has been active research into optical imaging apparatuses which irradiate light onto a living organism, which is an object under inspection, from a light source, such as a laser, and create an image of information inside the living organism obtained on the basis of the incident light. One example of optical imaging technology of this kind is Photoacoustic Tomography (PAT). In PAT, pulse light generated from a light source is irradiated onto a living organism, and the living tissue absorbs the pulse light which propagates and diffuses inside the organism and generates an acoustic wave (typically, an ultrasound wave), which is detected. The mechanism producing this photoacoustic wave is called the photoacoustic effect. An acoustic wave generated by the photoacoustic effect is called a photoacoustic wave.
A site under inspection, such as tumor, often has a higher light energy absorption rate compared to the surrounding tissue, and therefore absorbs a larger amount of light than the surrounding tissue and swells momentarily. The acoustic wave generated by this swelling action is detected by a transducer to obtain a reception signal. By mathematically analyzing the reception signal, it is possible to create an image of the acoustic pressure distribution of the photoacoustic wave produced by the photoacoustic effect inside the object. On the basis of photoacoustic image data obtained in this way, it is possible to obtain a distribution of optical characteristics, and in particular, a distribution of the absorption coefficient, inside the living organism. This information can be used for quantitative measurement of specific substances in the object, for example, glucose or hemoglobin contained in blood. In recent years, in use of the PAT, preclinical research which creates images of blood vessels of small animals, and clinical research which applies this principle to diagnosis of breast cancer, and the like, has been pursued actively.
FIG. 7A shows a schematic diagram of an image information obtaining apparatus which creates images of the interior of an object by PAT, and FIG. 7B shows a sound pressure-time curve of a photoacoustic wave arriving at the transducer shown in FIG. 7A. Furthermore, FIG. 7C shows a reception signal-time curve detected by the transducer, and FIG. 7D shows a photoacoustic image. The portion indicated by 101 in FIG. 7B is the signal amplitude caused by the photoacoustic wave occurring due to light absorption by the object, and is principally constituted by a low frequency component. Since the photoacoustic wave signal produced by light absorption in the object rises in the vicinity of the surface of the object, then in the present invention, the photoacoustic wave occurring due to absorption of light by the object is called a surface photoacoustic wave. On the other hand, the portion 102 is the signal amplitude caused by the photoacoustic wave produced by a locally situated optical absorber inside the object such as a tumor, and is principally constituted by a high frequency component. Since a sound pressure-time waveform is detected by a transducer of limited bandwidth having low sensitivity at low frequency, then a reception signal-time waveform such as that shown in FIG. 7C is obtained. The portion 103 in FIG. 7C is the transient response (signal amplitude) which occurs when the surface acoustic wave is detected by a transducer of limited bandwidth.
When the interior of an object is imaged by using the reception signal-time waveform in FIG. 7C and a photoacoustic image is acquired, the signal amplitude caused by the surface photoacoustic wave appears as an artifact such as that shown in FIG. 7D. Therefore, if an optical absorber, such as a tumor, is present nearer to the transducer side than the example shown in FIG. 7A, then the signal amplitude 102 caused by the photoacoustic wave produced by the optical absorber is concealed by the signal amplitude 103 caused by the surface photoacoustic wave. As a result of this, there is a problem in that an image of an optical absorber, such as a tumor, is concealed in the artifact, when imaging is performed.
Moreover, there are cases where a holding plate is provided in an image information obtaining apparatus which uses a photoacoustic effect. The holding plate is a mechanism which fixes an object on the apparatus. The purpose of providing this mechanism is to prevent movement of the object and change in the measurement position during measurement, and to enable imaging in a deep part of the object by making the object thinner by compression. FIG. 8A shows a schematic drawing of an image information obtaining apparatus of this kind; FIG. 8B shows a reception signal-time waveform and FIG. 8C shows a photoacoustic image.
If a holding plate is provided as in FIG. 8A, a multiply reflected surface photoacoustic wave is detected by the transducer, due to multiple reflection inside the holding plate. In other words, a waveform corresponding to 101 in FIG. 7 is reflected by the light source side surface of the holding plate, and is then reflected again by the object side surface, and detected by the transducer. In this case, a transient response similar to that described above is produced. This is the signal amplitude caused by the multiply reflected surface photoacoustic wave and is detected as shown in FIG. 8B. If a photoacoustic image is acquired using the reception signal including the multiply reflected surface photoacoustic wave, then an image such as that shown in FIG. 8C is obtained, and similarly to the case of the surface photoacoustic wave, there is a problem in that an image of a tumor, or the like, is concealed by an artifact produced by the multiply reflected surface photoacoustic wave.
Another issue similar to that of a surface photoacoustic wave described above in an image information obtaining apparatus using a photoacoustic effect is the appearance of artifacts in an ultrasound diagnostic apparatus. More specifically, multiple reflection of the transmitted ultrasound wave is repeated at intermediate objects between the transducer and the object, for example, the acoustic window, the object compression plate, or the like, and this manifests itself as multiple echo artifacts in the image.
A technique for reducing artifacts caused by multiple reflection of this kind in an ultrasound echo method is disclosed in Patent Literature 1. In the apparatus described in Patent Literature 1, a reference acoustic signal including a multiply reflected ultrasound wave is previously acquired by measurement using a phantom (artificial living organism), and the reference acoustic signal is then subtracted from the signal obtained by measuring the object.
Furthermore, Patent Literature 2 discloses a method for removing multiple echo produced by an acoustic window constituting an ultrasound diagnostic apparatus. In Patent Literature 2, multiple echo extracted by averaging a plurality of reception signals that have been received is subtracted from the reception signals so as to remove the multiple echo.
Moreover, Patent Literature 3 discloses a method for removing a multiply reflected image produced by a plate for compressing an object, which forms part of a medical imaging apparatus that displays an ultrasound diagnostic image. In Patent Literature 3, image data representing a plurality of ultrasound images is generated and a multiply reflected image is extracted from the generated image data. A multiply reflected image is removed by subtracting an extracted multiply reflected image from the image data of the object.